This invention relates generally to the field of light-emitting displays. More particularly, the invention relates to a novel cavity-emission electroluminescent device and a method for forming such a device.
Electroluminescent devices have become increasingly important within the display industry. Such devices are usually constructed in a multilayer thin-film or xe2x80x9csandwichxe2x80x9d configuration comprising a layer of electroluminescent material interposed between electron-injection and hole-injection electrode layers. When a voltage is applied to the electrode layers, holes and electrons are injected into the electroluminescent material from the hole injection and electron-injection electrode layers, respectively. Once the holes and electrons are combined in the electroluminescent material, light is emitted through one of the electrode layers. These electroluminescent devices have been described, e.g., in U.S. Pat. No. 5,247,190 to Friend et al., U.S. Pat. No. 5,682,043 to Pei et al., U.S. Pat. No. 5,723,873 to Yang and in Baigent et al. (1994), xe2x80x9cConjugated Polymer Light-Emitting Diodes on Silicon Substrates,xe2x80x9d Appl. Phys. Letter, 65(21):2636-38.
In the past, electroluminescent devices have used relatively small organic molecules as the electroluminescent material. See e.g., U.S. Pat. No. 4,539,507 to Van Slyke et al. However, much interest has recently been shown in the use of conjugated polymers in electroluminescent devices. For example, there is a present need for flexible displays. In order to produce a flexible electroluminescent device using an ordinary multilayer thin film configuration, each layer must have mechanical properties that can withstand the stresses and strains associated with deformation due to flexing. Thus, polymeric electroluminescent materials are preferred over relatively simple electroluminescent molecules. Electroluminescent polymers have been described in detail in a number of patents and publications such as U.S. Pat. No. 5,962,631 to Woo et al., International Patent Publication No. WO 98/27136 and Horhold et al., (1997), xe2x80x9cNovel Light Emitting and Photoconducting Polyarylene Vinylene Derivatives Containing Phenylene Arylamine and Phenylene Oxide Units in the Main Chain,xe2x80x9d Synthetic Metals, 84:269-70.
Ordinarily, the display configuration as described above requires at least one of the electrodes to be transparent with respect to the emitted light. See, e.g., U.S. Pat. No. 5,869,350 to Heeger et al. This is problematic because there are very few transparent materials that exhibit sufficient electrical conductivity to serve as an electrode material. Indium tin oxide (ITO) is one such optically transparent electrode material. However, the conductivity of ITO is more than an order of magnitude lower than that of most metals and is thus not an optimal material for display applications requiring a fast electroluminescent response. In addition, ITO lacks chemical stability for certain electroluminescent display applications. Moreover, because ITO is only a semitransparent material, ITO represents a source of internal device reflection when used as an electrode material in a multilayer configuration as described above.
In addition, multicolored displays using the above-described configuration are relatively difficult and expensive to produce due to inherent materials and processing limitations. In such a configuration, the electroluminescent layer serves to ensure that the electrode layers are electrically insulated from each other. Thus, the electroluminescent layer must be pinhole free. In addition, given the electronic properties of typical electroluminescent materials, the electroluminescent layer must have a uniform thickness of about 1000 to 2000 angstroms. This uniformity requirement is problematic because there is no known technique that can reliably and inexpensively produce color pixels in a uniform manner over a large area electroluminescent layer. While it may be possible to photolithographically pattern colors on electroluminescent polymers via photoresists, photoresist processing may adversely affect the uniformity in the thickness of the electroluminescent layer. Moreover, electroluminescent emission by the pixels in a direction parallel to the layers reduces the contrast between pixels. In turn, display resolution is compromised.
Some have proposed device configurations other than the above described layered or xe2x80x9csandwichxe2x80x9d structure. For example, U.S. Pat. No. 5,677,546 to Yu describes another configuration in which a light-emitting electrochemical cell may be made. Comprising an anode, a cathode and an electroluminescent film, the cell is constructed in a surface cell configuration, i.e., the anode and the cathode are in electrical contact with the same side of the electroluminescent film. The patent states that the electrodes may be formed using masking techniques such as photolithography. As another example, Smela et al., (1998), xe2x80x9cPlanar microfabricated Polymer Light-Emitting Diodes,xe2x80x9d Semicond. Sci. Technol., 13:433-39 describes an alternative device configuration. This article reports that diodes having interdigitated electrodes may be capable of electroluminescence even if the electrodes are separated by a distance of tens of microns. However, both these device configurations suffer from low brightness as the actual light emission zone is only a small fraction of the device area.
There is accordingly a need in the art to for an electroluminescent device that is capable of overcoming the inherent limitation of prior device configurations to provide a lightweight, high-efficiency and bright display device.
The present invention is addressed to the aforementioned need in the art, and provides a novel, low-cost, lightweight and easily fabricated cavity-emission electroluminescent device.
It is another object of the invention to provide such a display device that exhibits superior contrast, resolution and high power efficiency.
It is still another object of the invention to provide such a device that is suitable for a multicolor display.
It is a further object of the invention to provide a method for forming such a device.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention.
In one embodiment, then, the invention relates to an electroluminescent device. The device is comprised of a layered structure having a hole-injection electrode layer for injecting holes into an electroluminescent material, an electron-injection electrode layer for injecting electrons into an electroluminescent material and a dielectric layer interposed between the hole-injecting and electron-injecting electrode layers. A cavity extends through at least the dielectric layer and one of the electrode layers and has an interior cavity surface comprising a hole-injection electrode region, an electron-injection electrode region and a dielectric region. An electroluminescent coating material is provided in electrical contact with the hole-injection and electron-injection electrode regions of the interior cavity surface.
In another embodiment, the invention relates to a method for forming an electroluminescent device. The method involves providing a layered structure as described above having a cavity with an interior surface comprising a hole-injection electrode region, an electron-injection electrode region and a dielectric region. In one aspect, an etchant may be used to etch through a portion of a preformed layered structure to form the cavity. In another aspect, the layered structure may be formed around a sacrificial member having a desired size and shape for the cavity, wherein the sacrificial member is later removed to expose the interior cavity surface. Once the cavity is formed, the interior cavity surface is coated with an electroluminescent coating material such that the electroluminescent material electrically contacts the hole-injection and electron-injection electrode regions of the surface.
In a further embodiment, the invention relates to an electroluminescent display device. The display device comprises a layered structure as described above containing a plurality of cavities each extending through at least two layers of the layered structure such that each cavity has an interior cavity surface comprising a hole-injection electrode region, an electron-injection electrode region and a dielectric region. An electroluminescent material is provided in electrical contact with the hole-injection and electron-injection electrode regions of each interior cavity surface. Preferably, the plurality of cavities is arranged in an array.